Display devices using liquid crystal compounds are used widely at present for not only watches and electronic calculators, but also office automation equipment such as word processors and desktop computers, the automotive navigation system, etc. by dint of their low voltage driveability, very small electric power consumption and compactness as well as thin structure.
Liquid crystal display devices in general use make use of the nematic liquid crystal. The nematic liquid crystal is usually driven in the twisted nematic (TN) mode. It, however, is a drawback of the liquid crystal element driven in the TN mode that the driving margin becomes narrower with an increase in the number of scanning lines, with the result that a sufficient contrast becomes unachievable, and hence, it is difficult to fabricate large-capacity liquid crystal devices. Although the super twisted nematic (STN) mode has been introduced so as to improve such TN mode liquid crystal display device, the STN mode nonetheless poses such problem that both the contrast and response time deteriorate with an increase in the number of scanning lines.
What have attracted a great interest in place of the nematic liquid crystal affording only a slow response time are smectic liquid crystals such as ferroelectric liquid crystals and antiferroelectric liquid crystals. In the display device utilizing these liquid crystals the interaction between the spontaneous polarization inherent to the liquid crystal molecule and the intensity of the applied electric field generates the effective energy to change the direction of molecular orientation of the liquid crystal molecules, and consequentially, the response time is shortened and a high-speed response involving a switching time in the order of microseconds can be attained.
With a liquid crystal element fabricated by sealing the ferroelectric liquid crystal in a cell of a several .mu.m thickness (surface-stabilized ferroelectric liquid crystal element), two stable states can be secured for an electric field, as described in a technical paper of N.A. Clark et. al. (Appl. Phys. Lett., 36,899 (1980); authors: N. A. Clark and S. T. Lagerwall). The switching time in the electric field between these stable states is very short, namely, in the order of several microseconds. In the case of the antiferroelectric liquid crystal, three stable states prevail, and the tristable switching in this case is also very fast.
With the conventional nematic liquid crystal affording only a slow response speed, there were no other means but the active matrix drive (such as thin-film-transistor (TFT) operation) and the multiline addressing technique (super twisted nematic (STN) operation) to cope with problems associated with the driving. On the other hand, it is an advantage of the ferroelectric liquid crystal and antiferroelectric liquid crystal affording high response speeds that a simple matrix driving technique can be employed.
With regard to the viewing angle of displays, while nematic liquid crystals require an optically compensated film and a special device structure, it is the benefit of smectic liquid crystal that it dispenses with such special provisions.
In order to use such smectic liquid crystals in the display element, there are required such properties as a short response time and a stable contrast in the display device as well as an operating temperature range remaining in the vicinity of room temperature. At present it is difficult to fulmil all of the said properties by a single kind of liquid crystal, and hence a liquid crystal cell is usually prepared by blending several different kinds of liquid crystal. Particularly, as for the response time, a switching speed in the order of 10 some microseconds is requisite.
In order to shorten the response time of the ferroelectric liquid crystal material, it is necessary to increase the spontaneous polarization or to decrease the viscosity.
In antiferroelectric liquid crystal material, it is known that there is a relationship between the threshold voltage and the response time. The lower threshold time is required to shorten the response time.
However, in conventional ferroelectric liquid crystal material, there is a tendency that the static interaction of liquid crystal compounds each other is enlarged according to the increase of spontaneous polarization and consequently the viscosity is increased.
Notwithstanding the aforesaid requirements, the conventional liquid crystal element using antiferroelectric liquid crystal material in most cases has a cell gap of about 2 .mu.m and cells used for those liquid crystal element were operable with threshold voltage required to electrooptically change such elements in a range of 20-30 V/2 .mu.m in terms of an absolute value. Considering the fact that the ordinary complementary metal-oxide-semiconductor (CMOS) circuitry is operable at or below 15 V, it is difficult to drive a cell by a CMOS circuitry in a crystal liquid crystal element into which a cell requiring a threshold voltage of such a large absolute value is incorporated.
Additionally, the lower the said threshold voltage is, the larger becomes a deviation from the driving voltage. That is to say, the effective voltage increases with a decline in the threshold voltage. Accordingly, a lower threshold voltage is preferable for the reason that it serves to increase the electrooptical response of a display element. In the light of the aforesaid conditions, there are ardently awaited introduction of such antiferroelectric liquid crystal elements that have a practicably lowest absolute value of threshold voltage, for example, at or below 15 V/2 .mu.m and are capable of driving the cell at a large effective voltage.